Fix panic in SabreSwap with classical bits

This commit fixes a bug introduced in SabreSwap that was caught by the
randomized testing in Qiskit#8635. A copy paste error was causing the rusty
sabre code to panic in cases where there were classical bits of a
particular index assigned prior to the qubit with the same index. This
commit fixes the typo so the behavior is corrected in general and the
panic is also fixed.

Fixes Qiskit#8635

src/sabre_swap/sabre_dag.rs

@@ -54,7 +54,7 @@ impl SabreDAG {
             }
             for x in cargs {
                 if clbit_pos[*x] != usize::MAX {
-                    dag.add_edge(NodeIndex::new(qubit_pos[*x]), gate_index, ());
+                    dag.add_edge(NodeIndex::new(clbit_pos[*x]), gate_index, ());
                 }
                 clbit_pos[*x] = gate_index.index();
             }

test/python/transpiler/test_sabre_layout.py

@@ -19,7 +19,9 @@
 from qiskit.transpiler.passes import SabreLayout
 from qiskit.converters import circuit_to_dag
 from qiskit.test import QiskitTestCase
+from qiskit.compiler.transpiler import transpile
 from qiskit.providers.fake_provider import FakeAlmaden
+from qiskit.providers.fake_provider import FakeKolkata
 
 
 class TestSabreLayout(QiskitTestCase):
@@ -99,6 +101,43 @@ def test_6q_circuit_20q_coupling(self):
         self.assertEqual(layout[qr1[1]], 12)
         self.assertEqual(layout[qr1[2]], 11)
 
+    def test_layout_with_classical_bits(self):
+        """Test sabre layout with classical bits recreate from issue #8635."""
+        qc = QuantumCircuit.from_qasm_str("""
+OPENQASM 2.0;
+include "qelib1.inc";
+qreg q4833[1];
+qreg q4834[6];
+qreg q4835[7];
+creg c982[2];
+creg c983[2];
+creg c984[2];
+rzz(0) q4833[0],q4834[4];
+cu(0,-6.1035156e-05,0,1e-05) q4834[1],q4835[2];
+swap q4834[0],q4834[2];
+cu(-1.1920929e-07,0,-0.33333333,0) q4833[0],q4834[2];
+ccx q4835[2],q4834[5],q4835[4];
+measure q4835[4] -> c984[0];
+ccx q4835[2],q4835[5],q4833[0];
+measure q4835[5] -> c984[1];
+measure q4834[0] -> c982[1];
+u(10*pi,0,1.9) q4834[5];
+measure q4834[3] -> c984[1];
+measure q4835[0] -> c982[0];
+rz(0) q4835[1];
+""")
+        res = transpile(qc, FakeKolkata(), layout_method="sabre", seed_transpiler=1234)
+        self.assertIsInstance(res, QuantumCircuit)
+        layout = res._layout
+        self.assertEqual(layout[qc.qubits[0]], 14)
+        self.assertEqual(layout[qc.qubits[1]], 19)
+        self.assertEqual(layout[qc.qubits[2]], 7)
+        self.assertEqual(layout[qc.qubits[3]], 13)
+        self.assertEqual(layout[qc.qubits[4]], 6)
+        self.assertEqual(layout[qc.qubits[5]], 16)
+        self.assertEqual(layout[qc.qubits[6]], 18)
+        self.assertEqual(layout[qc.qubits[7]], 26)
+
 
 if __name__ == "__main__":
     unittest.main()